THE SIZE OF ATOMS. 197 



beautiful and interesting experiment to show you 

 the artificial "blue sky," produced by a very 

 wonderful effect of light upon matter, which he 

 discovered. We have here an empty glass tube 

 it is " optically void." A beam of electric light 

 passes through it now, and you see nothing. Now 

 the light is stopped, and we admit vapour of 

 carbon disulphide into the tube. There is now 

 introduced some of this vapour to about 3 inches 

 pressure, and there is also introduced, to the 

 amount of 15 inches pressure, air impregnated 

 with a little nitric acid, making in all rather less 

 than the atmospheric pressure. What is to be 

 illustrated here is the presence of molecules of 

 substances produced by the decomposition of 

 carbon disulphide by the light. At present you 

 see nothing in the tube ; it still continues to be, 

 as before the admission of the vapours, optically 

 transparent : but gradually you will see an ex- 

 quisite blue cloud. That is Tyndall's " blue sky." 

 You see it now. I take a Nicol's prism, and by 

 looking through it I find the azure light coming 

 from the vapours in any direction perpendicular 



